Variables — the foundation of any experiment
Independent (you change), dependent (you measure), controlled (kept constant).
Three types:
Independent variable (IV).
- What you DELIBERATELY CHANGE.
- The 'cause' you're testing.
- Plotted on X-axis.
Dependent variable (DV).
- What you MEASURE.
- The 'effect' caused by the IV.
- Plotted on Y-axis.
Controlled variables (CVs).
- Factors kept CONSTANT.
- Ensure a FAIR TEST — any change in DV must be due to IV (not other factors).
Example: Effect of temperature on amylase activity.
- IV: temperature (you change it).
- DV: time to break down starch (you measure it).
- CVs: enzyme concentration, starch concentration, pH, volume, mixing.
Why fair test matters.
- If multiple things change at once, you can't tell what caused the result.
- Each experiment should have ONE independent variable only.
Worked qualitative. A student investigated photosynthesis rate at different light intensities, but used a different plant for each test. Why is this BAD experimental design?
- Different plants vary in size, age, health → different photosynthesis baseline.
- This becomes a CONFOUNDING variable.
- Should use SAME plant (or identical plants) for each test.
Cambridge tip. When asked to identify variables, give all THREE types: IV, DV, CVs. Cambridge marks each.
- IV: change. DV: measure. CV: control.
- X-axis: IV. Y-axis: DV.
- Fair test = one variable changing.
- Confounding = uncontrolled variables ruining test.